Google added a new feature “data validation” in spreadsheets that lets you control what people enter in your spreadsheet? Once a range of cells are selected, the feature can be accessed in Tools > Data Validation and lets you create rules for numbers, text, and dates. A few notes on this new feature:

• If data validation is applied to cells containing data, rules won't be applied until the data is modified.
• Only one validation rule can be applied to each cell.

More info: Example